The process of malignancy induction provides close insights into more general cellular functions through a group of potentially transforming genes, termed protooncogenes. Two such genes, neu and kit structurally resemble well-characterized receptors for growth/differentiation factors. Although their ligands and physiological roles are yet unknown, their biology suggests involvement in the regulation of growth of mesenchymal and secretory tissues (neu) and stem cell differentiation (kit). Here we propose a multi-approach study designed to provide mutually independent, and perhaps complementary information on the functional roles of the neu and kit-encoded receptors. First, a series of sensitive bioassays will be employed to detect and later also to isolate the putative polypeptide growth factors that bind to the presumed receptors. Second and in parallel, anti-receptor monoclonal antibodies will be generated. Receptor-stimulatory antibodies will be selected and used to substitute the cognate ligands. Thirdly, chimeric receptors will be constructed in which the cytoplasmic portion of the presumed receptor will be linked to an extracellular domain of an established receptor. This configuration will allow us to study the cellular consequences of receptor activation by using an available heterologous ligand. Yet, a fourth approach to receptor physiology will utilize natural mutants: A carcinogen-induced mutation of the neu gene that appears to mimic binding of the putative ligand, and mutations that apparently involve the kit gene (in W mutant mice) and yield stem cell diseases. These mutations will be examined biochemically or through bone-marrow transplantation to get independent insights into the function of each gene. Combinely, these studies are expected to yield the identification of new growth factors and provide a comprehensive picture of the cellular pathways elicited by them. In addition to the relevance to human adenocarcinomas (neu) granulocytic anemia and leukemia (kit) we hope that the presented study will pave conceptual and methodological ways towards functional studies of the many other receptor-like oncogenic proteins.